Hydrocarbon (e.g., oil, natural gas, etc) in a subterranean formation can be recovered by drilling a well into the subterranean formation. Hydrocarbon in the subterranean formation is driven into the well to be produced by, for example, pressure gradients that exist between the formation and the well, the force of gravity, displacement of the fluids using pumps or the force of another fluid injected into the well. The production of hydrocarbon is commonly increased by hydraulically fracturing the subterranean formation. In hydraulic fracturing, a viscous fracturing fluid is pumped into a well at a rate and pressure sufficient to form fractures that extend into the formation, providing additional pathways through which the fluids can flow into the well and be recovered.
The fracturing fluid is usually a water-based fluid containing a gelling agent to increase the viscosity of the fluid. The gelling agent reduces the leakage of liquid from the fractures into the subterranean formation. The gelling agent is commonly a polymeric material that absorbs water and forms a gel as it undergoes hydration.
In addition, particulate matter known as proppant, e.g., graded sand, bauxite, or resin coated sand, is typically dispersed throughout the fracturing fluid. Fracturing fluid has been developed for its proppant carrying capacity, fluid loss control, and high viscosity. The proppant is suspended in the fracturing fluid and becomes deposited into the fractures created by the pressure exerted on the fracturing fluid. The presence of the proppant in the fractures holds the fractures open after the pressure has been released. Otherwise, the fractures would close, rendering the fracturing operation useless. Ideally, the proppant has sufficient compressive strength to resist crushing.
Foamed fracturing fluid is one type of fracturing fluid that has a relatively large volume of gas dispersed in a relatively small volume of liquid. Foamed fracturing fluid also includes a surfactant for facilitating the foaming and stabilization of the foam produced when the gas is mixed with the liquid. The most commonly used gases for foamed fracturing fluids are nitrogen and carbon dioxide because they are non-combustible, readily available, and relatively cheap.
The procedure used to fracture a subterranean formation with foamed fracturing fluid includes pumping the foamed fracturing fluid into a well at a pressure sufficient to fracture the formation and then relieving the pressure at the wellhead to reduce the pressure of the fracturing fluid, or defoam the fracturing fluid. Upon the defoaming of the fracturing fluid, the proppant suspended therein is released into the resulting fractures. This reduction of the pressure on the foam is referred to as “breaking” the foam, which results in the fluid being carried back into the well by a rush of expanding gas. However, controlling the defoaming or breaking of the fracturing fluid to properly release the proppant can be difficult.
One type of foamed fracturing fluid has been developed that is advantageously controlled by change of pH. Thus, upon lowering the pH of the pH-dependent foamed fracturing fluid, the foam is broken, or defoamed, which results in the fluid being carried back into the well by a rush of expanding gas. Such pH-dependent, foam fracturing fluids are further discussed in Halliburton's U.S. patent application Ser. No. 10/396,606, filed Mar. 25, 2003, entitled “Recyclable Foamed Fracturing Fluids and Methods of Using the Same;” Halliburton's Pub. No. US 2005/0043188, entitled “Recyclable Foamed Fracturing Fluids and Methods of Using the Same”; Halliburton's Pub. No. US 2005/0077047 entitled, “Methods of Fracturing a Subterranean Formation Using a pH Dependent Foamed Fracturing Fluid”; and Halliburton's Pub. No. US 2004/0200616 entitled, “Recyclable Foamed Fracturing Fluids and Methods of Using the Same,” all of which are incorporated by reference in their entirety.
A method is needed to defoam the pH-dependent foamed fracturing fluid after the hydraulic fracture treatment is completed.